Space launch vehicles, particularly those that are defined as "upper stage" i.e., boosted to a high altitude by one or more "booster" rockets must be highly efficient since their weight and propellants must be carried throughout the mission. Liquid hydrogen is often used in space launch vehicles since it is highly efficient as a propellant when burned with an oxidizer, typically liquid oxygen. High performance turbopumps typically used as primary propellant pumps on upper stage rocket engines to pump the liquid hydrogen and oxygen are sensitive to a characteristic known as net positive suction pressure ("NPSP"). Since the liquid hydrogen and liquid oxygen have boiling point temperatures far below typical ambient temperatures they are continuously boiling. This boiling may be suppressed by increasing the NPSP at the primary pump inlet. One method of increasing NPSP is by pressurizing the propellant tanks and using boost pumps installed at the propellant tank exits. The boost pumps provide the additional pressure necessary at the primary pump inlets during engine start-up, thereby suppressing boiling of the propellants.
FIG. 1 shows a schematic diagram of a typical conventional rocket propulsion system 10 of the prior art. The system includes propellant tanks for liquid hydrogen 11 and liquid oxygen 12 with their associated boost pumps 13, 14 located at the tank exit, typically within the tank volume. Each boost pump 13, 14 is typically driven by a turbine 15, 16 located external to the tank 11, 12. The propellant supply lines 17, 18 transfer propellants from the boost pumps 13, 14 to the primary pumps 19, 20 through inlet valves 21, 22. A regeneratively cooled thrust chamber 23 vaporizes the liquid hydrogen flowing to the thrust chamber through the main propellant line 24. Prior to injection into the thrust chamber, the gaseous hydrogen may be expanded through a primary turbine 25 to drive the primary pumps 19, 20. In some applications, a portion of the gaseous hydrogen may be diverted from the main propellant line 24 and ducted through a bypass line 26 and a check valve 40 to one of the propellant tanks 11 to maintain pressure in the tank 11 as the propellant flows out.
Alternatively, stored gas such as high pressure helium from one or more helium tanks 27 may be used for tank pressurization, in which case it is necessary to reduce the pressure and control the flow of the stored gas using orifices 28 or controllers.
Each boost pump turbine 15, 16 is typically powered by an auxiliary power system including monopropellant stored in a monopropellant tank 29. The monopropellant decomposes in the presence of a catalyst to release energy to drive the turbine 15, 16. However, since the monopropellant tank 29 must be pressurized to force flow through the turbines, the stored gas 27 must typically be used for this purpose. Thus the use of monopropellant adds significant weight and complexity to the rocket engines of the prior art.
What is needed is an auxiliary power system that reduces the weight and complexity of the monopropellant system of the prior art.